.box-list {
    width: 200px;
    .box {
        width: 100%;
        height: 100px;
        margin-bottom: 10px;
        background-color: blue;
        &:nth-child(3n) {
            background-color: blue;
        }
        &:nth-child(3n + 1) {
            background-color: red;
        }
        &:nth-child(3n + 2) {
            background-color: green;
        }

    }
}